High intensity light emitting devices are interesting for various applications including spot lights and digital light projection. For this purpose it is possible to make use of so-called light concentrators where shorter wavelength light is converted to longer wavelengths in a highly transparent luminescent material. A rod of such a transparent luminescent material is illuminated by (e.g. blue) LEDs to produce longer wavelengths within the rod. Converted light (e.g. red, yellow or green, dependent on the composition of the luminescent rod) will be in the luminescent material in the waveguide mode if the luminescent material is sufficiently transparent, the surfaces of the rod are perfectly polished and the ambient has a significantly lower refractive index. The waveguided light can then be extracted from one of the exit surfaces leading to an intensity gain. The light output can be increased by incoupling of more light by making the waveguide longer and adding LEDs.
Because the light from the LEDs is converted inside the luminescent rod towards longer wavelengths, part of the impinging energy from the blue LEDs is converted into heat inside the luminescent rod, due to the involved Stokes shift. Therefore the luminescent rod will heat up during operation. The performance and efficiency of the light conversion process is dependent on the temperature of the luminescent rod, as thermal quenching, optical saturation levels and quantum efficiency are temperature dependent. This temperature dependence is largely determined by the exact material composition and e.g. doping levels of the luminescent rod. As a variety of materials are used for the luminescent rod, such as Ce doped YAG- and LuAG-like crystals, the performance of the high intensity light emitting device may show a very strong decrease with temperature. Therefore it is crucial to keep the luminescent rod temperature below a given threshold temperature, which may e.g. be 150° C. for a LuAG system (Green light source), or even lower for e.g. YGdAG systems (Orange/Red light source).
As single-sided illumination is less attractive than double-sided to achieve high light output intensities and as three-sided or four-sided illumination is hard to embody using LED-boards, the luminescent rod is typically illuminated from two opposite sides, leaving, perpendicular thereto, two opposite surfaces free to apply cooling.
In one approach the luminescent rod and the LEDs of such a high intensity light emitting device can be cooled each by its own cooling element, typically a heat sink or heat pipe. This, however, would result in four cooling interfaces to the surroundings, which in turn results in an excessively complex system. Consequently, such a system has turned out not to be appealing to customers.
Furthermore, it is desired to provide both optimum total internal reflection (TIR) at all 6 interfaces between the luminescent rod and the ambient, and optimum cooling of the luminescent rod from two opposite sides. On the one hand, to obtain optimum total internal reflection (TIR) at the interfaces between the luminescent rod and the ambient, a gap between the luminescent rod and the ambient having a thickness of more than 2 times the wavelength is required such as to obtain little or preferably no optical contact between the luminescent rod and the ambient. On the other hand for optimum cooling of the luminescent rod from two sides good thermal conductance to a cooling element, typically a heat sink or heat pipe, is required, and to obtain a high thermal conductance, C=k/d, the distance d between the luminescent rod and the heat sink must therefore be as small as possible.
U.S. Pat. No. 8,525,999 B2 describes a light emitting diode illumination system comprising a LED die with a central luminescent rod. Two high thermal conductivity boards are arranged on mutually opposite sides of the LED die. The LED die is cooled by means of two heat sinks arranged one on each of the two thermally conductive boards opposite to the LED die. The thermally conductive boards may e.g. be copper or aluminum core printed circuit boards. The connection between LED die, thermally conductive boards and heat sinks is not described any further.
Providing some kind of spacing element arranged between the respective heat sinks and the luminescent rod, such as e.g. the thermally conductive boards of U.S. Pat. No. 8,525,999 B2, construes an attempt at solving the problem of providing both optimum TIR at the interfaces between the luminescent rod and the ambient, and optimum cooling of the luminescent rod on two sides. However, such high intensity light emitting devices are complex devices and therefore also expensive to manufacture.